This invention relates to titanium alloy/fiber composite materials. In particular, this invention relates to a method for improving the microstructure of such composite materials.
In recent years, material requirements for advanced aerospace applications have increased dramatically as performance demands have escalated. As a result, mechanical properties of monolithic metallic materials such as titanium alloys often have been insufficient to meet these demands. Attempts have been made to enhance the performance of titanium by reinforcement with high strength/high stiffness filaments or fibers.
Titanium matrix composites have for quite some time exhibited enhanced stiffness properties which closely approach rule-of-mixtures (ROM) values. However, with few exceptions, both tensile and fatigue strengths are well below ROM levels and are generally very inconsistent.
These titanium composites are typically fabricated by superplastic forming/diffusion bonding of a sandwich consisting of alternating layers of metal and fibers. At least four high strength/high stiffness filaments or fibers for reinforcing titanium alloys are commercially available: silicon carbide, silicon carbide-coated boron, boron carbide-coated boron and silicon-coated silicon carbide. Under superplastic conditions, which involve the simultaneous application of pressure and elevated temperature, the titanium matrix material can be made to flow without fracture occurring, thus providing intimate contact between layers of the matrix material and the fibers. The thus-contacting layers of matrix material bond together by a phenomenon known as diffusion bonding. Unfortunately, at the time of high temperature bonding a reaction can occur at the fiber-matrix interfaces, giving rise to what is called a reaction zone leading to lower mechanical properties. The compounds formed in the reaction zone may include reaction products like TiSi, Ti.sub.5 Si, TiC, TiB and TiB.sub.2. The thickness of the reaction zone increases with increasing time and with increasing temperature of bonding.
Titanium matrix composites have not reached their full potential, at least in part, because of problems associated with instabilities of the fiber-matrix interface. The reaction zone surrounding a filament introduces sites for easy crack initiation and propagation within the composite, which can operate in addition to existing sites introduced by the original distribution of defects in the filaments. It is well established that mechanical properties of metal matrix composites are influenced by the reaction zone, and that, in general, these properties are graded in proportion to the thickness of the reaction zone.
Several methods have been proposed for reducing, if not eliminating the interfacial reactions. Friedrich et al, U.S. Pat. No. 3,991,928, disclose that interfacial reactions between reinforcing silicon coated boron fibers and commercially available rolled beta titanium alloy foil can be substantially eliminated by consolidating a stack of fiber-reinforced foils with an applied pressure in excess of 22 ksi. and a temperature of about 1250.degree. to 1275.degree. F. Smith et al, U.S. Pat. No. 4,499,156, disclose that interfacial reactions between a variety of reinforcing fibers and titanium alloy foils can be substantially eliminated by extensively cold working the alloy to obtain a sheetstock having a grain size less than 10 microns, then consolidating a stack of these cold-worked foils with interspersed fibers with an applied pressure of 10 to 100 MPa and a temperature about 10.degree. to 100.degree. C. below the beta-transus temperature of the alloy. More recently, Eylon et al, in U.S. patent applications Ser. No. 935,362 and Ser. No. 935,363, both filed Nov. 26, 1986, and Ser. No. 936,679, filed Dec. 1, 1986, disclose methods for preparing titanium alloy composite structures which comprise the use of rapidly solidified titanium alloy foils and consolidation with an applied pressure of 1.5 to 15 Ksi and a temperature below the beta-transus temperature of the alloy.
The matrix microstructure of a consolidated composite is a very fine equiaxed alpha structure, the result of the large amount of alpha+beta deformation during compaction, i.e. superplastic forming/diffusion bonding, as well as the compaction thermal cycle which is carried out in the alpha+beta phase field. While the fiber-reinforced matrix has better tensile strength than the unreinforced metal, the very fine equiaxed titanium alpha microstructure of a consolidated composite has low fracture resistance and low creep strength. The fracture resistance and creep strength of non-fiber-reinforced titanium alloys can be improved by heat treating the alloy at a temperature above its beta-transus temperature, which results in a lenticular alpha plate morphology with excellent fracture and creep resistance. The fracture resistance and creep strength of a consolidated composite can be improved after compaction by similar heat treatment which products a matrix with lenticular alpha plate microstructure. Such heat treatment cannot be done prior to fabrication of the composite because the matrix material will not flow unless it has the equiaxed alpha morphology. On the other hand, it is undesirable to heat treat a composite after compaction, because of the development of interfacial reactions between the reinforcing fiber and the titanium alloy matrix at higher temperatures.
Accordingly, it is an object of the present invention to provide a method for improving the microstructure of titanium alloy metal matrix composites.
Other objects and advantages of the present invention will be apparent to those skilled in the art.